Grafting of monomers onto polyolefins is well known (see `Polymer Chemistry` by M. P. Stevens, (Addison-Wesley), 1975, pp. 196-202). Maleation is a type of grafting wherein maleic anhydride is grafted onto the backbone chain of a polymer. Maleation of polyolefins falls into at least three subgroups: maleation of polyethylene, maleation of polypropylene, and maleation of copolymers of propylene and ethylene or other monomers.
Maleation of polyethylene provides higher molecular weight products with a noticeable decrease in melt index due to cross-linking, unless special provisions are made, (see for example `Journal of Applied Polymer Science`, 44, 1941, N. G. Gaylord et al (1992); and U.S. Pat. Nos. 4,026,967; 4,028,436; 4,031,062; 4,071,494; 4,218,263; 4,315,863; 4,347,341; 4,358,564; 4,376,855; 4,506,056; 4,632,962; 4,780,228; 4,987,190; and 5,021,510). Maleation of polypropylene follows an opposite trend and yields lower molecular weight products with a sharp increase in flow rate due to fragmentation during the maleation process (see for example U.S. Pat. Nos. 3,414,551; 3,480,580; 3,481,910; 3,642,722; 3,862,265; 3,932,368; 4,003,874; 4,548,993; and 4,613,679). Some references in the literature fail to note the difference between maleation of polyethylene and polypropylene, and claim maleation of polyolefins with conditions which are useful only for either polyethylene or polypropylene, respectively. In general, conditions which maleate polypropylene are not ideal for maleation of polyethylene due to the opposite nature of the respective maleation chemistries: fragmentation to lower molecular weights for polypropylene and cross-linking to higher molecular weights for polyethylene. This is shown in U.S. Pat. No. 4,404,312. Maleation of copolymers of propylene and ethylene or other monomers follow the pattern of the majority component.
Maleations of polypropylene can also be further subdivided into batch or continuous processes. In batch processes all of the reactants and products are maintained in the reaction for the entire batch preparation time. In general, batch maleation processes cannot be used competitively in commerce due to high cost. Batch processes are inherently more expensive due to startup and cleanup costs.
The maleated polypropylene's that are reported in the previous literature can also be divided into two product types as a function of whether or not solvent is involved, either as a solvent during reaction or in workup of the maleated products. In U.S. Pat. Nos. 3,414,551; 4,506,056; and 5,001,197 the workup of the product involved dissolving the maleated polypropylene product in a solvent followed by precipitation, or washing with a solvent. This treatment removes soluble components and thus varies both the `apparent` molecular weight and the acid number. Processes using an extruder produce a product in which solvent soluble components remain. In addition, extruder processes often incorporate a vacuum system during the latter stages of the process to remove volatile lower molecular weight components. Thus different compositions are necessarily present in products produced in an extruder in contrast to those products from solvent processes or those which use a solvent in product workup.
Another subdivision of maleation of polyolefins concerns the state of the reaction process. Solvent processes, or processes where solvent is added to swell the polypropylene (see U.S. Pat. No. 4,370,450), are often carried out at lower temperatures than molten polyolefin (solvent free) processes. Such processes involve surface maleation only, with substantial amounts of polypropylene below the surface being maleation free. Processes using molten polypropylene involve random maleation of all of the polypropylene. Solvent processes are also more expensive in that solvent recovery/purification is necessary. Solvent purification is even more expensive if the process inherently produces volatile by-products, as in maleation. Note that if water is the `solvent`, polypropylene is not soluble and reaction must occur only on the surface of the polypropylene solid phase. Further, in aqueous processes maleic anhydride reacts with the water to become maleic acid. In these two ways processes containing water are necessarily different from non-aqueous processes. In a molten process no solvent or water remains at the end of the process to be purified or re-used. Thus a molten process would be environmentally `greener` and less expensive.
Present commercial maleation of low flow rate (high molecular weight) polypropylene by continuous processes, such as in an extruder, produce products with acid numbers well below 4. These products are used in adhesives, sealants, and coatings and as couplers and compatibilizers in polymer blends. However, due to the low acid numbers, the adhesion and coupling properties of these maleated polypropylenes are limited. As noted above, attempts to produce higher acid number polypropylene in continuous processes yield higher colored products with much lower molecular weight with maleic anhydride conversion efficiencies of 20-30% or lower (see for example U.S. Pat. No. 5,001,197). Attempts to produce higher acid number polyethylene in continuous processes yield cross-linking, higher color, and gels (see for example U.S. Pat. Nos. 4,612,155; 4,639,495; 4,751,270; 4,762,890; 4,857,600; and 4,927,888). The patent literature does describe continuous maleation of high flow rate (low molecular weight) polypropylene waxes to higher acid numbers. However, as noted above the molecular weights of the maleated waxes so produced are even lower than that of the starting material due to fragmentations during maleation.
In light of the above, it would be very desirable to maleate lower flow rate polypropylenes in a continuous process to higher molecular weights and higher acid numbers with lower colors than have been known before. It would also be very desirable to maleate these polypropylenes at higher efficiencies.